This invention relates generally to communication networks and more particularly to systems for qualifying telephone lines for data transmission.
As is known in the art, public switch telephone networks, i.e., so-called plain old telephone service (POTS) lines, were originally designed for voice communications that cover a limited frequency bandwidth (i.e., about 4 KHz). Today, it is desired to use the same POTS lines for data transmission. Data signals, however, generally have different frequency characteristics than voice signals, including bandwidths that are orders of magnitude larger. As a result, a POTS line that works well transmitting voice signals might not work well, or at all, for data signals. Telephone companies need to know which lines are suitable, i.e., qualified, and which lines are not suitable for data transmission. Telephone companies also need to know why particular lines are unable to support data transmissions and where such faults occur so they can determine whether the transmission line can be corrected.
Line Qualification is the overall ability to make statements about the quality of a subscriber loop as it relates to its ability to deliver voice communications (i.e. POTS), or data services. Of particular interest herein is the qualification of lines to support high speed data transmission, such as ISDN, SDSL, ADSL or HDSL. Pre-disqualification is the ability to make a statement with a high degree of confidence that a subscriber loop will not support a data service without remedial actions. Pre-qualification is the ability to make a statement with a high degree of confidence that a subscriber loop will support a data service.
Telephone operating companies (TELCO's) have two problems to solve in qualifying subscriber loops for delivery of data. The first problem is strategic. Telco's are reluctant to deploy emerging technologies for the delivery of data (e.g., ISDN or ADSL) because there is uncertainty in their knowledge that sufficient subscriber loops are of high enough quality to make deployment economically successful. This discourages early adopters because there is significant risk in being first to deliver a technology that may not work in their access network. If Telco's could be given a technology to take much of this risk out of initial deployment, they can secure market share and lead in the face of competition.
The second problem is tactical and comes after a Telco has made a decision to deploy a particular technology. There is a need to qualify, either pro-actively or reactively, specific lines for service as that service is requested by subscribers or targeted by the Telco for delivery, or the Telco may be compelled by regulation to provide sufficiently capable lines to CLEC (Competitive Local Exchange Carrier). For example, if a Telco were to market and deliver the new service, they would like to target those subscriber loops most likely to support the service out of the box and/or with a minimum of work. As another example, a Telco receiving a new service request from a subscriber desires information to either accept or reject that request for new service based on the condition of their line.
Line qualification is generally done with single-ended or double-ended testing. For double ended testing, a technician is dispatched to each end of the line under test. The line being tested is disconnected from the network and test equipment is installed at both ends of the line. The test equipment cooperates to determine whether the line supports the required type of data transmission.
For xDSL line qualification, one two-ended approach currently in use is to dispatch one technician to the upstream side of the line under test, such as at the Central Office (CO) or Digital Loop Carrier (DLC) and another technician to the subscriber side of the line. The technicians isolate the line under test and then connects a series of special purpose instruments to both ends of the line, such as a load coil detector or a time domain reflectometer to detect bridge taps or a digital multimeter to locate resistance faults. This approach gives only an incomplete indication of signal loss on the line—which we have recognized is a very important predictor of line performance—and is too time consuming and expensive to use for wide spread deployment of a particular type of high speed data service.
Various techniques have been proposed to make two ended measurements without the need to deploy a person at the far end of the line at the time of the measurement. For Example, U.S. Pat. No. 5,402,073 to Ross, entitled “Near-End Communications Line Characteristic Measuring System With Voltage Sensitive Non-Linear Device Disposed at the Far End,” describes a non-linear element at the far end of each line to aid in making end to end measurements. U.S. Pat. No. 6,091,713 to Lechleider et al. entitled “Method and System for Estimating the Ability of a Subscriber Loop to Support Broadband Services” describes the use of an analog modem at the far end of a line to predict performance. However, if a device needs to be attached to the line to conduct a test, a human must be present at the far end of each line at some time in order to make the test setup. Further, if the device is moved or not properly set up, the test might yield incorrect results—making it difficult to rely on such tests for widespread testing or qualification. U.S. Pat. No. 6,215,855 to Schneider, entitled “Loop Certification and Measurement for ADSL” is one example of data processing that can be done with two ended measurements. U.S. Pat. No. 6,177,801 to Chong, entitled “Detection of Bridge Tap using Frequency Domain Analysis” is another example.
Double-ended testing is not desirable because of the time and cost associated with having test equipment at both ends of the line. Additionally, double-ended testing often provides results that are specific to a particular type of data service so that the testing has to be repeated if the type of data services changes.
Others have tried to do single-ended xDSL pre-qualification using single ended measurements. One approach is to use automated measuring equipment developed for testing lines for faults that effect voice service and relying on records telephone companies keep of their lines to provide additional information. The cable information can indicate if the line has been configured in a way that is known to be unsuitable for a particular data service. For example, records of the wire gauge of the line and whether a load coil is installed might indicate that the line will not support a certain type of service. However, the review of the cable records has generally been done manually, resulting in unacceptable long test times. In addition, telephone companies have generally found that the required records are inaccurate, because the required information was entered incorrectly or never updated. Moreover, line characteristics that impact speed of digital data services did not necessarily cause any degradation of traditional voice services. For many years, there was no reason for a telephone company to keep records of the information they would now need to qualify lines for high speed data services. Therefore, the cable records often do not contain the required information.
Some have proposed calculation techniques that allow single-ended measurements to qualify a line. For example, U.S. Pat. No. 5,864,602to Needle, entitled “Qualifying Telephone Line for Digital Transmission Service” is one example. However, it would be desirable to make the qualification as accurate as possible.
Additionally, some techniques for pre-processing of telephone lines have been described. For example, U.S. Pat. No. 6,111,861 to Burgess, entitled “Method and System for managing High Speed Data Communication” describes a system in which line conditioning is varied to either allow or block high speed data services.
However, as a result of the limitations of presently available line pre-qualification techniques, phone companies are not able to predict with a high level of confidence whether their lines will support certain types of data services. These limitations have forced service providers to restrict their offering of high speed data services to particular regions where they believe the lines are capable of supporting them or to only promise customers that the services will operate at less than their full possible speed.
A particular source of problems for telephone companies wishing to offer high speed data services is bridged taps. A bridged tap is a pair of wires connected to a telephone subscriber line at one end and un-terminated at the other. Bridged taps are usually wires that were part of an initial layout of the subscriber loop in anticipation of the loop being connected to a particular subscriber premises. But, either because the subscriber premises were not constructed in the place anticipated or the loop was rewired after installation, some un-terminated wire was left attached to the loop. For voice services, the bridged tap has little impact. However, for high speed data services, we have recognized that the bridged tap might have an impact that varies from significant enough to prevent transmission of high speed data services to virtually none at all.
Thus, it would be very useful for a telephone company to understand whether repairing a bridged tap (such as by removing it) would actually allow a subscriber line to operate at a desired speed.
If a telephone company could determine, quickly and inexpensively, which lines support high speed data services and the speed at which those lines will operate, it would be a significant advantage for that company. Further, it would be a significant advantage if the telephone company could more accurately determine whether a repair on a line would allow it to support a required level of service.